1. Field of the Invention
The present invention relates to a process for making a high temperature-resistant ceramic material with an adjustable thermal expansion coefficient .alpha..sub.30-1400.degree. C. in the range of that of platinum and its alloys, namely from 9.times.10.sup.-6 K.sup.-1 to 15.times.10.sup.-6 K.sup.-1, and method of use of this ceramic material.
2. Prior Art
Magnesia materials have been known for a long time as fire-resistant materials, especially in the form of firebrick or refractory brick, and have attained great importance in the last 30 years, e.g. as furnace brick in the glass industry, as working lining in pig iron mixers. They are also used in rotary kilns or furnaces in the cement industry.
The features of magnesia brick and thus its manufacture are determined by the particular application involved. Conventional bricks have a high cold compressive strength, a low porosity and soften under load in oxidizing atmospheres over about 1400.degree. C.
In general their resistance to changes in temperature is not very good, since the thermal expansion at 1400.degree. C. can already reach 2%.
Considerable improvements may be attained here by using flux-poor, especially iron-poor, sintered magnesia as a raw material, by variations of the grain structure, of the degree of packing or bulk factor or also by additives, for example spinel (MgAl.sub.2 O.sub.4) or chromium ore. The proportion of magnesia spinel brick has clearly increased in recent years in relation to the costs of the chrome ore containing embodiments because of environmental problems.
A composite body comprising one part metallic titanium and another part ceramic substance based on the magnesium oxide-aluminum oxide-titanium dioxide system is described in DE-AS 1 471 174. This composite body is characterized by an at least partially chemical binding between titanium and ceramic. This is clearly necessarily due to the content of titanium dioxide.
The composite body may contain no identifiable silicic acid-containing ceramic. The composite body is used from making vacuum tubes. The process described in DE AS 1 471 174 produces the composite body by a mixed precipitation. In contrast, the ceramic material according to the present invention is made exclusively by sintering and/or fusion processes.
A ceramic material made from at least two metal oxides in which the metals have two different valences is, for example, disclosed in European Patent Document EP 0 739 864 A1, in which spinel is the main phase and the oxide of the metal with the lower valence is present in the spinel matrix above the stoichiometric ratio of 1:1 and dissolved in it. The metallic part of the first oxide is magnesium and the metallic part of the second oxide is aluminum.
This type of ceramic material according to EP 0 739 864 A1 is employed in the chemical industry, the metals industry and the ceramic industry, where for example alkaline media at elevated temperatures enclosed in a gas and/or liquid-tight vessel containers must be conducted in pipes or must be separated by it.
In this type of material however the amount of magnesium oxide present must be small enough so that the MgO in the spinel remains dissolved and only the spinel is identified as a single phase by X-ray photographic methods.
DE 195 18 468 A1 discloses use of a fire-resistant cast body based on a hydrated ion-sensitive metal oxide, such as MgO, with a carbon content, a dispersing agent and a reactive silicic acid for coating or repair of the erosion-endangered regions of a metallurgical smelting vessel.
Other literature relating to the MgO/Al.sub.2 O.sub.3 -Spinel material, include for example:
"MAGNESIO-ALUMINATE SPINEL--A POTENTIAL RAW MATERIAL FOR MAKING NEW GENERATION REFRACTORIES" by B. Ghosh, P. Chakraborty, P. G. Pal, S. K. Mitra and K. S. Swaminathan; Tata Refractories Limited, India, published in "Global Development of Refractories", Proc. Of the UNITECR '95 Congress, Kyoto, Japan, Nov. 19 to 22, 1995, pp. 541 to 549.
"SINTERED MgO CLINKER CONTAINING Al.sub.2 O.sub.3 ", of Kaneyasu, Akira and Shimmatsu, Satoshi, Technical Div., Ube Chemical Ind. Col. Ltd., Ube, Japan, and Shima, Hiromi, Faculty of Engineering, Yamaguchi, Univ., Ube, Japan, published in "Global Development of Refractories", Proc. of the UNITECR '95 Congress, Kyoto, Japan, Nov. 19 to 22, 1995, pp. 550 to 557.
"PERIKLAS-SPINEL PRODUCTS WITH IMPROVED PROPERTIES BY TARGETED TiO.sub.2 ADDITIVES" by Wolfgang Schulle, Ph. Gia Khanh and Vu Tuan Anh, published in Veitsch-Radex Rundschau 1-2, 1995, pp. 563 to 567.
"MECHANICAL PROPERTIES OF MODEL MAGNESIA-SPINEL COMPOSITE MATERIAL" by C. Aksel, R. W. Davidge, P. D. Warren and F. L. Riley, School of Materials, University of Leeds, Leeds LS 2, 9JT, UK, published in Key Engineering Materials, Vols. 132 to 136 (1997), pp. 1774 to 1777; 1997 Trans. Tech. Publications, Switzerland.
None of the above literature disclose or suggest the purpose and solution of the present invention.
Components made from refractory ceramics and metal-coated refractory ceramics are used in glass smelting furnaces and in hot forming of glass bodies. The fire-resistant ceramic "supporting" materials currently used have thermal expansion coefficients of about 0.5.times.10.sup.-8 K.sup.-1 (quarzal) to about 8.times.10.sup.-6 K.sup.-1 (sillimanite, mullite, .alpha.-alumina) in a temperature range of from 30 to 1400.degree. C. These values are clearly lower than those for platinum and Pt alloys with .alpha..sub.30-1400.degree. C. of 11.5.times.10.sup.-6 K.sup.-1. A frequent cause of failure of these components has been shown by damage control analysis to be faults and fractures in the platinum casing which arise because of the different thermal expansion coefficients of both materials.